An extensive radio system for alerting the public to national and regional emergencies has been implemented. For example, broadcasts of the National Oceanic and Atmospheric Administration (N.O.A.A.) wheather radio stations transmitting at about 162 MHz are adapted to broadcast emergency messages. In the N.O.A.A. emergency system, an emergency message is preceded by a broadcast of a 1.05 kHz tone.
The limitations of the N.O.A.A. emergency broadcast system as well as other broadcast systems, are essentially twofold. First, the frequency to which the radio receiver should be set varies geographically. Thus, as the radio receiver travels from place to place, in the case of an automobile radio, it is necessary to retune the radio to pick up the N.O.A.A. broadcast at the new frequency. Even where the location of the radio is fixed, there may be a need to retune the receiver. For example, if a relatively close N.O.A.A. transmitter goes off the air, it may be necessary to tune to a more distant N.O.A.A. transmitter at a different frequency. While variations in the frequency to which the receiver should be tuned is not a problem when the receiver is being monitored, it is a problem where the receiver is unmonitored.
The second limitation mentioned above stems from the need to monitor the radio receiver to not only retune, if necessary, but also to detect the presence of the "alert" tone preceding an emergency broadcast. Insofar as nonemergency N.O.A.A. broadcast repeat the same information over a fairly short interval, a listener will not generally desire to continuously listen to a N.O.A.A. broadcast. Also, of course, an emergency broadcast can occur during the late evening or early morning hours when most of the population is asleep. Thus, it can be expected that a relatively small percentage of the population would be alerted to an emergency condition by the N.O.A.A. weather radio network or any emergency broadcast system.
The above-described limitations of the emergency broadcast system, as it is presently configured, have been addressed by radio receivers monitoring a broadcast station in an inactive mode and then switching to an active mode when an "alert" tone, indicative of an emergency condition, is transmitted. Thus, U.S. Pat. No. 4,031,467, to Singleton, Jr., et al., discloses a radio alerting system including a master station and several receivers. The receivers are normally inactive, but they become active upon receiving respective predetermined control tones modulating the received signal. The master station selects receivers for receipt of a message by selectively modulating the transmitter signal with appropriate control tones.
U.S. Pat. No. 3,701,024, to Knowles et al., discloses a radio warning system in which the output of a receiver is connected to a detector that detects a predetermined number of pulses at a predetermined frequency and then closes a gate to apply the audio output of the receiver to a power amplifier and loudspeaker. The Knowles et al. system also selects between two different receivers, depending upon whether one of the receivers is detecting a broadcast signal.
U.S. Pat. No. 3,124,749, to Craig, discloses an emergency broadcast control device that applies predetermined tones and a blanking signal to a broadcast signal.
These prior attempts to improve the usefulness of the emergency broadcast system do not address the problem of locating the frequency at which the broadcast is transmitted while a receiver is unattended. Nor do the prior art systems address the problem of being too expensive to be placed in widespread use or of being too high in power consumption to operate over a long period of time on battery power. Other limitations inherent in the prior art systems will become apparent in comparison with the inventive emergency broadcast alert detector described herein.